The familiar Snellen eye testing chart uses large black, serifed letters on a white background and is the universal standard by which visual resolution is compared. A disadvantage of the Snellen chart is the fact that defocused letters can still be partially recognized by their blur patterns; much time is thus wasted as the patient whose eyes are being tested attempts to guess the letter. The design of the Snellen chart is further complicated by the fact that each letter has a different degree of recognizability.
Analysis of the spatial frequency content of the Snellen letters reveals that they contain both high and low spatial frequencies in generous measure. The blurring of the letters is attributable to lack of response to the patient's eye to the higher frequency components in the letters. However, the patient is still capable of responding to the lower frequency components by correctly identifying the blur pattern.
It is therefore an object of this invention to provide letters to be used in conjunction with an eye testing chart in which the low spatial frequencies of the letters have been minimized.
The letters used in conventional eye testing charts are discussed at length in pages 380-385 of the book, "Clinical Refraction", 3rd Edition, I. M. Borish, Professional Press, Inc., Chicago, Illinois. It is seen from this reference that the primary consideration in the prior art was the selection of groups of letters having nearly equal difficulty of recognition. Reference is made therein to an article by Walker, J. P. S., Oph. (1942): Test Type, B. J. Oph., 25:555. Page 385 of the reference book shows Walker's letters to be comprised of letters formed by a 5.times.5 checkerboard matrix, the letter strokes so formed being either completely white or black with the opposite color of approximately the same area filling the voids of the letter to provide a square black letter with a gray background.
It is an object of this invention to provide a letter chart where the letters of a given size are of substantially equal recognizability so that the choice of a letter on a line of letters of a desired difficulty of recognition need not be restricted as in the prior art to a limited group of letters. Therefore, it is a feature of this invention that the number of different charts is limited only by the number of permutations of substantially all the letters in the alphabet (except for the letters I, M, and W which represents width extremes) and if desired of numbers. Thus, the number of permutations of the letters is substantially greater than that possible with prior art charts in which the letters of a particular line are chosen to be of equal recognition difficulty.
It is therefore an object of this invention to provide letters which have substantially less distinct blur patterns and less recognizability when not in focus than those in the prior art while those same letters which are in focus are easily recognizable.
It is a further object to provide letters which shift in and out of focus more critically with changes in letter size or viewing distance than the prior art letters.
It is a further object of this invention that the letters blend into the gray background rather than becoming partially recognizable blurs when the letters are too far defocused to be recognized without guessing.
It is a further feature of this invention that in one embodiment the letters of the eye testing chart are outlined in black with the interior portions of the letter being white on a gray background having the same reflectance as the average reflectance of the black and white areas of the letter. Alternately, the letter can be outlined in white with the inner portion being black on a gray background, the reflectance being similarly determined.
It is a further feature of one embodiment of this invention that there may be interposed on the gray background of the chart between the letters a plurality of black or white dots, asterisks, or stars to provide a "noise" background which will inhibit recognition of letters of marginal visibility.
It is a further feature of this embodiment that the line stroke used to form the letters can instead be used to form a single and easily recognized figure, such as a cat, a chair, or cup, and that different width line strokes can be similarly outlined to form a plurality of such figures so that children and other illiterates can be tested.